The invention relates to a novel strain of Streptomyces sp., designated Streptomyces candidus Y21007-2; a conditioned medium of the strain; the uses of the medium; a method for producing borrelidin utilizing the strain; and the novel uses of borrelidin.
Fungal phytopathogen is one of the causes leading to severe economic losses in the agricultural and horticultural industries. Infected plants show inhibited germination and growth, and soft rot, or die. In addition, soils infested with phytopathogenic fingi may be unsuitable for growing certain crops.
Phytophthora spp. is an important soil-borne fungal phytopathogen, which causes diseases to a broad host range, e.g. stalk and leaf lesions of melons and fruits. The infected parts of plants exhibit water-infiltrated lesions at the initial stage. The lesions will rapidly turn brown, and rotten. If the moisture remains high, a large amount of white fungal mass will grow and result in rotten, hollow fruits with brown spots. The seedlings of plants will die immediately after being infected with Phytophthora spp. Once infected, the stems and leaves of plants above the ground will turn yellow and harden, or wilt.
Pythium spp. is another important soil-borne fungal phytopathogen, which causes diseases to the seedlings of Cruciferae spp. and melons. Seeds at the emerging stage infected with the pathogen will not emerge or become rotten. Infected seedlings will exhibit water-infiltrated lesions at the infected parts, wilt and die. For the damping-off seedlings, the infected tissues are brown, dried, thinned, and stunted.
Currently, fungicides widely used for treating Phytophthora and Pythium infections are chemical agents, including Etridiazole (also known as Terrazole), Previcur N (also known as Propamocarb hydrochloride), Metiram (also known as Rinclozolin), Ridomil (also known as Metalzaxyl), Mertect (also known as Thiabendazole), Oxine-Copper (also known as Quinolate), Dithane M-45 (also known as Mancozeb) and Dithane Z-78 (also known as Zineb).
Due to the intensive use, the strains resistant to some of the above chemical fungicides have been reported, e.g. Phytophthora megasperma f. sp. medicaginis tolerant to Ridormil (see R. M. Hunger et al., Plant Dis. 66:645-649 (1982)).
Moreover, the environmental protection has drawn the international attentions. In viewing that the abuse of chemical fungicides is ecologically detrimental, there is a need to develop environmentally acceptable biological agrochemicals.
Borrelidin is a macrolide antibiotic, which was first isolated by Berger et al. in 1949 from the culture medium of Streptomyces rochi and reported to be active against borrelia, the relapsing fever spirochete, and capable of enhancing the activity of penicillin G against syphilis spirochete. See J. Berger et al., Arch. Biochem. Biophys. 22:476-478 (1949).
Borrelidin-producing strains and new uses of the antibiotic have been subsequently identified in the art. For instance, M. Lumb et al., Nature 206:263-265 (1965) describes borrelidin produced by S. griseus C2989 strain as an antiviral agent which is inactive against bacteria; V. Prikrylova et al., Physiological Aspects 34:422-423 (1989) reports a new strain of S. rochi which produces borrelidin as a substance inhibiting germination of plant seeds; and DE 36 07 287 A1 discloses borrelidin produced by a new S. griseus strain as a pesticide and herbicide.
U.S. Pat. No. 4,759,928 discloses an antibacterial material produced by S. albovinaceus which is active against Treponema hyodysenteriae. The antibacterial material is latter identified as borrelidin. See H. Maehr et al., The Journal of Antibiotics, Vol. XL No. 10, pp. 1455-1456 (1987).
Borrelidin has been chemically characterized and identified as 2-[7-cyano-8,16-dihydro-9,11,13,15-tetramethyl-18-oxooxacyclooctadeca-4,6(E, Z)-diene-2-yl]cyclopentacarboxylic acid with the following structure: 
See M. Kuo et al., The Journal of Antibiotics, Vol XLII NO. 6, pp. 1066-1077 (1989).
The above references and patents, however, do not disclose the antifungal activity of borrelidin, and are herewith incorporated as the references of the application.
A new Streptomyces candidus strain, designated Streptomyces candidus Y21007-2, has been surprisingly found. The conditioned medium of the strain is found to be active against fungi and capable of promoting the germination of seeds.
Therefore, in the first aspect of the invention, the novel strain Streptomyces candidus Y21007-2 is provided.
In the second aspect of the invention, the conditioned medium of the novel strain is provided.
In a further aspect of the invention, the uses of the above conditioned medium as a fungicide and as a seed germination stimulator are provided.
The active ingredient in the conditioned medium of Streptomyces candidus Y21007-2 has been identified as borrelidin.
Accordingly; in a still further aspect of the invention, there is provided a method for producing borrelidin which comprises cultivating the novel strain Streptomyces candidus Y2 1007-2.
The invention also provides the novel uses of borrelidin as a fungicide and as a seed germination stimulator.